1. Field of the Invention
This invention relates to an image pickup apparatus that is utilizable to such as a camera, and particularly it relates to a reading process for image-pixel signals generated by an image sensor.
2. Description of the Related Art
As for a digital camera with an image sensor, it is required to produce a subject image by a wide dynamic range. For example, when an image sensor is used as a light-metering sensor or AE sensor, the brightness of an object should be precisely detected between a low luminance level and a high luminance level to determine an appropriate exposure value such as a shutter speed.
Various methods for enlarging the dynamic range of an image sensor are known or proposed. For example, an exposure time is changed in each pixel, a plurality of images is acquired in time series, or an amount of incident light is changed in each pixel by dividing a light path. Also, it is known that pixels having low sensitivities and pixels having high sensitivities are mixture-arrayed. In this case, OB image-pixel signals that are read from OB (Optical Black) pixels having high sensitivity are utilized to calculate output values of OB image-pixel signals that are read from OB pixels having low sensitivity. Thus, values of image-pixel signals generated in a light-receiving area are corrected adequately.
On the other hand, a method that reads out pixel signals while mixing or adding pixel signals is known. For example, a light-metering sensor incorporated in an SLR type digital camera outputs image-pixel signals while adding pixels. JP2012-253462A1 discloses a camera having an AE sensor with a color filter array, in which R, G, and B color elements are arrayed in a stripe along a vertical direction of the AE sensor. Then, five pixel signals that are neighboring one another along the vertical direction are mixed in each of the R, G, and B color filter elements. The five mixed pixel signals of each color element are output as one pixel signal.
Such a pixel adding method improves the sensitivity in a low luminance range; however, a real effect that enlarges a dynamic range totally cannot be obtained since a pixel adding process is uniformly performed on the entire light-receiving area of the image sensor.
On the other hand, a metering sensor in the camera is utilized to pursue a subject by extracting characteristics of the subject, such as a face, and high resolution is required to extract the characteristics of the subject. In the above pixel adding method disclosed in JP2012-253462A1, to prevent a decrease in resolution the pixel adding process is not performed when pursuing a subject using the metering sensor. In this case, the sensitivity in the low luminance level is not increased. Also, the number of image-pixel signals to mix, i.e., the number of added pixels, is constant under any photography condition when performing the pixel addition process.
Furthermore, in the above pixel adding method disclosed in JP2012-253462A1, output levels of OB image-pixel signals corresponding to output levels of image-pixel signals generated by the pixel addition process are not considered sufficiently.